Focused light beams are employed in many optical fields, such as lithography, confocal microscopy, optical data storage, for example, or in optical particle traps. It is important in all of these applications to obtain the smallest possible focusing radius, which can be achieved only by the use of high-aperture optical systems. In this connection, it is increasingly important, however, to consider the polarization properties of the electromagnetic field. Thus, for example, in the case of a linearly polarized, high-aperture light beam, the energy distribution in the focus no longer has rotation symmetry to the optical axis, but is elliptically deformed. A particular shape of the intensity and polarization distribution in the focus can be achieved by specifically setting the polarization distribution within the cross-section of the light beam. In this case, radially or azimuthally polarized beams are of particular interest, because the former have a strong longitudinal electrical component near the focus, whereas in the case of the latter, the electrical field in the focal center disappears completely. It has been shown that the use of radially polarized light beams allows to generate the smallest possible spot diameters so far. Therefore, it is important to have optical elements which transform an unpolarized light beam into one having a well-defined polarization distribution.
Further important fields of application requiring a spot diameter which is as small as possible are microscopic wafer inspection as well as high-resolution material microscopy.
However, so far, it has been very complex to generate radially or azimuthally polarized light. It may be generated, for example, in the laser resonator by superimposing TEM01 and TEM10 Hermite Gauss modes being polarized orthogonally to each other, or in the beam path by the use of a Mach Zehnder-type interferometer. It is also possible to use mode-forming, holographic and birefringent elements.
In view thereof, it is an object of the invention to provide an optical element, which is suitable for selectively deflecting only one desired polarization, and, if required, to adjust the orientation of this polarization in the deflected beam according to a given path line in the beam cross-section.
The object is achieved by a polarization-selectively blazed, diffractive optical element comprising a plurality of contiguous blaze structures, which extend along a given geometrical path and each have a width perpendicular to their direction of extension, said width being greater than the wavelength of the electromagnetic radiation for which the diffractive optical element is designed, and each of said structures comprising a plurality of individual substructures, which are arranged next to each other according to a predetermined period in the direction of extension, said substructures providing the blaze effect and each having the shape, when viewed from above, of a closed geometrical surface whose dimension parallel to the direction of extension varies perpendicular to the direction of extension, but is always smaller than the wavelength of the electromagnetic radiation, and whose maximum dimension perpendicular to the direction of extension is greater than the wavelength of the electromagnetic radiation, wherein the filling ratio of the dimension of the individual structures in the direction of extension relative to the predetermined period is selected such, as a function of the position perpendicular to the direction of extension, that the blaze effect is optimized for one of two mutually orthogonal polarization conditions of the electromagnetic radiation.
The desired polarization selectivity of the diffractive optical element is achieved by the sub-wavelength pattern of the individual blaze structures along the direction of extension by means of the individual substructures. The electromagnetic radiation of only one polarization condition is predominantly directed into the predetermined blaze order, so that the blaze efficiency of one polarization condition is considerably greater than that of the other. In particular, the blaze efficiency is improved by more than 50% for one polarization condition as compared to the other. However, if the filling ratio is appropriately selected, the blaze efficiency for one polarization condition may even be more than twice as high and may even be one to several orders of magnitude better than for the other polarization condition.
Any statements made herein with regard to the direction of extension always refer to the respective local direction of extension. If the geometrical shape of the blaze structures is a circular ring shape, for example, then the radial direction is always perpendicular to the direction of extension, and the tangential direction is parallel to the direction of extension.
The blaze effect of the individual substructures is obtained because, due to the sub-wavelength pattern in the direction of extension, said pattern cannot be resolved directly by the electromagnetic radiation, so that it only sees a refractive index averaged via at least one (preferably more) individual substructures, said refractive index varying locally due to the geometrical shape of the individual substructures in a direction perpendicular to the direction of extension. This effective profile of the refractive index perpendicular to the direction of extension for the electromagnetic radiation is now adapted so as to achieve the desired blaze effect. In particular, the desired blaze effect can be achieved by a linear increase in the refractive index in a direction perpendicular to the direction of extension. However, this linear increase can usually be achieved only for one of two orthogonal polarization conditions, because averaging of the refractive index is effected differently for the different polarization conditions. Therefore, if the filling ratio is selected such that the blaze effect is optimized for one of two polarization conditions, which are orthogonal to each other, the blaze effect for the other of said two polarization conditions deteriorates at the same time (as compared to the case where the blaze effect is best for unpolarized light and, thus, for both polarization conditions at the same time).
Since the individual substructures, when viewed from above, have the shape of a closed geometrical surface, they and, thus, also the optical element can be easily manufactured, e.g. by means of known methods of semiconductor manufacture.
The individual substructures are usually arranged such that always just one individual substructure is provided in each blaze period perpendicular to the direction of extension. The individual substructures are thus arranged next to each other in the direction of extension.
The two orthogonal polarization conditions are preferably linear polarization conditions. If the blaze structures have a circular ring shape, for example, beams can thus be generated which are radially or azimuthally polarized.
In particular, the width of the individual blaze structures may be varied such that the diffractive optical element still has an imaging effect, i.e. acts as a lens. The width of adjacent blaze structures may decrease, increase, or decrease and then increase again, or increase and then decrease again. The width of the individual blaze structures along the direction of extension is preferably constant or varies randomly or with a statistic distribution around a mean value.
In a particularly preferred embodiment of the diffractive optical element according to the invention, the individual substructures comprise a first layer having a first refractive index and a second layer having a second refractive index that is different from the first refractive index, said second layer being arranged on the first layer. In particular, a third layer having the second refractive index is arranged between the individual substructures in the region of the first layer. This stacked configuration of the individual substructures allows the achievement of an extraordinarily high degree of polarization of the blaze order (ratio of the transmission of one optimized polarization condition of the desired blaze order to the sum of both polarization conditions of the desired blaze order). The degree of polarization may be 80% to 99.9%.
In particular, the second and third layers are equal in height. This considerably simplifies the production of the diffractive optical element, because only one further coating step needs to be carried out after forming the first layer of the individual substructures, if they are provided in an elevated form on the surface of a carrier, which further step then causes the layer thus applied to be located either as a second layer on the individual substructures or as a third layer between the individual substructures. Of course, if desired, a fourth layer may be applied on the third layer, said fourth layer having a refractive index which differs at least from that of the second layer.
It is further possible that the individual substructures may comprise a first layer having a first refractive index and a second layer having a second refractive index, which second layer is arranged on the first layer, and that further individual structures are arranged between the individual substructures, which further structures comprise a third layer having a third refractive index and a fourth layer arranged on the third layer, said fourth layer having a fourth refractive index. All of the refractive indices may be different. Further, all of the individual layers may have different heights. In particular, the individual structures are provided so as to fill the entire space between the individual substructures.
The filling ratio of the individual substructures which comprise the first and second layers is preferably selected such that it does not cover the entire range of from 0 to 1. The size of the range covered by the filling ratio is preferably not greater than 0.7, in particular not greater than 0.5. For the individual substructures comprising only one layer, the filling ratio may also be selected in the same manner.
The diffractive optical element may preferably be provided such that the individual substructures are contiguous perpendicular to the direction of extension of the blaze structures. Of course, they may also be spaced apart perpendicular to the direction of extension, which may depend, for example, on manufacture. The individual substructures may be contiguous or may be spaced apart in the direction of extension.
When viewed from above, the individual substructures may have the shape of a trapeze, or of any other quadrangle or polygon, in particular of a triangle, with at least one side thereof preferably having a curved shape. It is further possible to provide the individual substructures so as to respectively have symmetry to an axis perpendicular to the direction of extension.
At least one side of the individual substructures may be approximated by a stepped curve.
A preferred embodiment of the diffractive optical element according to the invention consists in that, in addition to a region comprising the individual substructures, the element comprises a further region including conventional blaze structures having an at least approximately ramp-shaped profile. If the blaze structures form an annular geometrical shape together with the individual substructures, the region comprising the conventional blaze structures is preferably arranged in the center of the element, i.e. where the blaze period (perpendicular to the direction of extension) is the greatest in blazed, diffractive optical elements for imaging.
The predetermined geometrical shape may be either a closed path (annular path) or an open path. The closed path may be a circular ring shape, an elliptic shape, a polygon shape or any other type of curved shape. The open path may comprise part of a self-contained path line, any other curved or polygon-type shape, or may even be linear. The width of the blaze structures (dimension perpendicular to the direction of extension) may be varied as a function of the position in the direction of extension in order to achieve a desired imaging property. If the blaze structures extend along closed path lines, the blaze structures may be arranged concentrically to each other.
The predetermined shape is selected, in particular, as a function of a desired imaging property of the element and/or a structured illumination which emits the electromagnetic radiation for which the element is optimized. The structured illumination may be, for example, a ring, dipole or quadrupole illumination.
The diffractive optical element is preferably a transmissive element.
The individual substructures may be provided with an elevated shape on the top surface of a carrier. However, it is also possible for the individual substructures to be embedded in a carrier (e.g. having a planar top surface) and to have a refractive index which differs from the refractive index of the laterally surrounding carrier material.
The individual substructures may be provided as doping zones or as depletion zones in the carrier material.
In particular, the diffractive optical element may be designed for electromagnetic radiation having a wavelength in the visible spectral region, in the infrared region or in the UV region.
Preceding or following the diffractive element, there may be arranged a phase element which causes a locally varying phase displacement in the electromagnetic beam incident on or coming from the diffractive element. The phase element is preferably a transmissive element which may be provided as a separate element or integrally with the diffractive element. If it is provided integrally with the diffractive element, it is preferably not provided on that side on which the blaze structures are formed.
For example, if the diffractive element comprises blaze structures having point symmetry, there may be out-of-phase or in-phase oscillations of the light fields of two points having point symmetry to the center of the beam cross-section of the beam transmitted by the diffractive element. The locally varying phase displacement is then selected such, for example, that these light fields, if desired, oscillate in-phase or out-of-phase, respectively. For this purpose, the phase element, for example, may be provided such that its thickness increases (in a spiral-shaped manner) as a function of the angular position, said thickness being selected such, in particular, that a phase delay of 2π is achieved after 360°. Of course, the phase element may also be provided such that it imposes other locally varying phase displacements on the beam.
Further, a polarization element may be arranged preceding the diffractive element, said polarization element pre-polarizing the electromagnetic beam for the diffractive element at least partially into said one orthogonal polarization condition. Thus, an already pre-polarized beam which predominantly has said one desired orthogonal polarization condition, is incident on the diffractive element. The polarization element is preferably not of the imaging type, so that it contributes, together with the diffractive element, if the latter is an imaging element, to a further improvement of the polarizing effect, whereas the imaging effect is caused exclusively or mainly by the diffractive element.
Further, an objective is also provided which contains at least one diffractive optical element according to the invention or a described further embodiment thereof. The objective may be, for example, a microscope objective or an objective used in semiconductor lithography and may comprise further lenses or optically effective elements, respectively.